Clock generating circuit and method

ABSTRACT

A clock generating circuit is provided that includes a multiplexing device coupled to a clock distribution network to select between a synchronous mode and an asynchronous mode. The device may also include a plurality of distributed ring oscillators to drive the clock distribution network in the asynchronous mode. In the synchronous mode, the multiplexing device may pass a signal from a phase lock loop circuit located external to a core.

FIELD

[0001] The present invention relates generally to microprocessor circuits, and more specifically to internal clocks in microprocessor circuits.

BACKGROUND

[0002] Electronic devices, such as microprocessors, are steadily operating at faster and faster speeds. As microprocessors run at higher and higher speeds, the power delivered to the microprocessors by a power supply starts to become an issue. Voltage drops (or droops) may occur as power is delivered from a power source to individual components and devices on the die of a microprocessor. For example, devices on a die may receive only 1.0 volt from a power source that is supplying 1.2 volts due to a voltage droop. Decoupling capacitors may be used on a die to help reduce voltage droop. However, decoupling capacitors cost area on the die and also cost power due to gate oxide leakage.

[0003] Power source voltage droops affect the speed at which an electronic device (e.g., microprocessor or integrated circuit) may operate. During normal operation of a microprocessor (or any sequential machine), noise may be generated from instantaneous switching. Voltage supply noise modulates the delay of data paths. Voltage droops reduce the maximum frequency of operation of the microprocessor. For example, as a voltage droop magnitude increases, the operating frequency of the microprocessor decreases. Thus, a large change in processor activity may cause substantial supply voltage transients resulting in performance loss.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The foregoing and a better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing example arrangements and embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and that the arrangements and embodiments are not limited thereto.

[0005] The following represents brief descriptions of the drawings in which like reference numerals represent like element and wherein:

[0006]FIG. 1 is a block diagram of an integrated circuit according to one arrangement;

[0007]FIG. 2 is a schematic diagram of a clock distribution network according to one arrangement;

[0008]FIG. 3 is a diagram of one stage of a clock distribution network according to one arrangement;

[0009]FIG. 4 is a block diagram of a clock generating circuit according to an example embodiment of the present invention;

[0010]FIG. 5 is a diagram of a clock generating circuit according to an example embodiment of the present invention;

[0011]FIG. 6 is a diagram showing the clock generating circuit of FIG. 5 along with elements to be clocked according to an example embodiment of the present invention;

[0012]FIG. 7 is a diagram of a clock generating circuit according to an example embodiment of the present invention;

[0013]FIG. 8 is a diagram of a clock generating circuit according to an example embodiment of the present invention;

[0014]FIG. 9 is a diagram of a clock generating circuit according to an example embodiment of the present invention;

[0015]FIG. 10 is a diagram of a start/stop circuit according to an example embodiment of the present invention; and

[0016]FIG. 11 is a graph showing a Vcc voltage signal that varies due to noise.

DETAILED DESCRIPTION

[0017] In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Further, in the detailed description to follow, example values may be given, although embodiments of the present invention are not limited to the same. While values may be described as HIGH or LOW, these descriptions of HIGH and LOW are intended to be relative to the discussed arrangement and/or embodiment. That is, a value may be described as HIGH in one arrangement although it may be LOW if provided in another arrangement. Arrangements and embodiments may be shown in block diagram form in order to avoid obscuring the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements and embodiments may be highly dependent upon the platform within which the present invention is to be implemented. That is, such specifics should be well within the purview of one skilled in the art. Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without, or with variation of, these specific details. It should also be apparent that differing combinations of hard-wired circuitry may be used to implement embodiments of the present invention. That is, embodiments of the present invention are not limited to any specific combination of hardware.

[0018] Embodiments of the present invention may also be described with respect to signals being input or output from different circuit components. It is understood that while the discussion identifies a signal, the signal may be transmitted over a signal line or similar type of mechanism. Further, the terminology signal may also correspond to a signal line as shown in the drawings. Well-known power/ground and address connections to components may not be shown within the figures for simplicity of illustration and discussion, and so as not to obscure the invention.

[0019] While the following discussion may be presented with respect to implementation in a microprocessor, embodiments of the present invention are not limited to that specific implementation. Implementations for generating clock signals for various digital devices such as integrated circuits, discrete logic devices, memory devices, devices either on the same or separate chips, communications devices, etc., are also within the scope of the present invention.

[0020] Embodiments of the present invention may provide a circuit that includes a clock distribution network and a multiplexing device coupled to the clock distribution network to select between a synchronous mode and an asynchronous mode. A plurality of distributed ring oscillators may asynchronously drive the clock distribution network in the asynchronous mode. The distributed ring oscillators may be coupled to a power supply such that they track the critical paths. A phase lock loop circuit (located external to the core circuit) may synchronously drive the clock distribution network in the synchronous mode. In the following discussion, the terminology asynchronously driving the clock distribution network may be used with reference to the asynchronous mode. In the asynchronous mode, the clock distribution network may be driven asynchronously relative to an external clock. The clock distribution network may be synchronously driven although it is asynchronous with respect to an external clock.

[0021] Embodiments of the present invention may thereby provide power supply control on a microprocessor. This allows performance to be recovered since the performance may be dependent on the average power supply level rather than minimizing power supply droop. The core clock frequency may instantaneously track the worst-case speedpath over Vcc noise. The instantaneous performance of the processor core may vary over time in response to Vcc transient.

[0022]FIG. 1 is a block diagram of an integrated circuit according to one arrangement. Other arrangements are also possible. More specifically, FIG. 1 shows an integrated circuit 100 having a core 101, an interface 105, and a clock generator 102. The core 101 may include circuitry and logic to perform the designated functions of the integrated circuit, while the interface 105 may provide an interface between the core 101 and the remainder of the system and its system bus(es). For instance, if the integrated circuit 100 includes a processor, the core 101 may include one or more decoders, scheduling logic, execution units, reorder buffers, memory order buffers, register files, cache memory, etc., for use in executing instructions. The interface 105 may include external bus controller logic and programmable interrupt controller logic.

[0023] The clock generator 102 may generate the clock signals in response to a system clock signal 110. The clock generator 102 may include a phase lock loop (PLL) circuit. The clock signals may be coupled to the core 101 and the interface 105. The clock generator 102 may generate the bus clock signal(s) 103 and the core clock signal(s) 104.

[0024]FIG. 2 illustrates a clock distribution network 200 according to one arrangement. Other arrangements are also possible. The clock distribution network 200 distributes a clock signal to chip components such as the core 101 (shown in FIG. 1). As illustrated, a feedback clock signal and a reference clock signal may be applied to a PLL 210, which may be provided within the clock generator 102. The clock distribution network 200 may include a plurality of drivers 220, 230, 240, 250, 260, 270 and 280 to drive large capacitances, such as attributable to registers and latches, with the output signal of the voltage-controlled oscillator of the phase lock loop (PLL). The drivers 220, 230, 240, 250, 260, 270 and 280 may contain inverters (not shown). Hence, the capacitances may be switched at the clock frequency. The capacitances 265, 275, 285, 290, 295 and 255 may be the capacitances attributable to the components of the chip. In addition to these capacitances, the gate capacitances of the driver inverters may also be switched at the clock frequency. If the total capacitance for the clock network is represented as C and the clock network switches at the clock frequency, f, the amount of power dissipated may be represented as CV²f, where V is the supply voltage. This amount of power may be a significant portion of the total power utilized by the chip due to a relatively large C and a relatively high f.

[0025]FIG. 3 is a diagram of one stage of a clock distribution network according to one arrangement. Other arrangements are also possible. More specifically, FIG. 3 shows one stage of a clock distribution network 300 that may be provided within the core 101 (FIG. 1). FIG. 3 shows the terminal stage with multiple drivers driving a common and continuous grid. The clock distribution network 300 may also be provided within other entities. The core clock distribution network 300 may include signal traces 310 (shown vertically in the drawing figure) and signal traces 320 (shown horizontally in the drawing figure). The core clock distribution network 300 may also include a plurality of drivers coupled to the signal traces 310 and 320. For ease of illustration, only a first driver 330 is labeled in FIG. 3. The drivers operate to provide clock signals (such as core clock signals) to the clock distribution network 300 and thereby provide clock signals to respective elements (such as latches and registers) of the core 101 (not shown in FIG. 3).

[0026]FIG. 4 is a block diagram of a clock generating circuit according to an example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. In this embodiment, the clock generating circuit may be provided within the core 101 although the clock generating circuit may also be located in other locations that include a clock distribution network. More specifically, FIG. 4 shows a clock generating circuit 400 coupled between a first signal trace 312 and a second signal trace 322. The first signal trace 312 may be one of the signal traces 310 and the second signal trace 322 may be one of the signal traces 320. However, embodiments of the present invention are also applicable to the signal trace 312 and the signal trace 322 being configured into a clock distribution network different from the clock distribution network shown in FIG. 3. FIG. 4 only shows one clock generating circuit although the plurality of clock generating circuits may be distributed throughout the clock distribution network.

[0027]FIG. 4 also shows a power supply (or power supply device) 350 provided external to the core 101, such as in a periphery (i.e., the I/O) of the integrated circuit about the core 101 (FIG. 1). The power supply 350 may be coupled by a power distribution network 355 to the clock generating circuit 400. Although not shown in FIG. 4, a plurality of clock generating circuits 400 may be provided about the clock distribution network in a similar manner as each of the drivers provided about the clock distribution network 300 in FIG. 3. In other words, the clock distribution network may include a plurality of clock generating circuits 400 each coupled between signal traces and each powered by the power supply 350 through the power distribution network 355.

[0028]FIG. 5 is a diagram of a clock distribution circuit according to an example embodiment. FIG. 5 shows more specific circuit elements (such as inverter circuits and a multiplexing device) each of which may be powered by the power supply 350 coupled via the power distribution network 355. As shown, a plurality of inverter circuits 410, 420, 440, 450 and 460 and a multiplexing device 430 (or selecting device) may be coupled as a ring oscillator (or ring oscillator circuit) between the signal trace 312 and the signal trace 322. That is, the inverter circuit 410 receives a signal from the signal trace 322. The signal propagates through the inverter circuit 420, through the multiplexing device 430, through the inverter circuits 440, 450 and 460 and is output to the signal trace 312 from the last stage of the ring oscillator (such as the inverter circuit 460). In other words, the input signal to the ring oscillator and the output signal of the ring oscillator are to the clock distribution network.

[0029]FIG. 5 shows five stages of a ring oscillator circuit coupled between signal traces. The ring oscillator circuit may include any odd number of stages so as to produce an oscillating circuit between signal traces. That is, while FIG. 5 shows five inverter circuits, embodiments of the present application are also applicable to other numbers of inverter circuits coupled in series so as to produce a ring oscillator circuit. Furthermore, circuit elements other than inverter circuits may also be used to form the ring oscillator.

[0030] The multiplexing device (or selecting device) 430 is coupled between the inverter circuit 420 and the inverter circuit 440. The multiplexing device 430 selects between inputs on a signal line 432 and a signal line 434. The multiplexing device 430 may receive an input signal on the signal line 434 from the inverter circuit 420. The multiplexing device 430 may receive input signals on the signal line 432 from a phase lock loop (PLL) circuit provided external to the core 101, for example. That is, the phase lock loop circuit may provide a clock signal along the signal line 432 to the multiplexing device 430.

[0031] The multiplexing device 430 may receive a select signal to select between an asynchronous mode and a synchronous mode. In the synchronous mode, the clock signal on the signal line 432 passes through the multiplexing device 430, and subsequently passes through the inverter circuits 440, 450 and 460. The resulting signal is output to the signal trace 312 (i.e., the clock distributing network). This thereby results in the clock distribution network operating based on a synchronous signal provided by the PLL located external to the core 101. On the other hand, the multiplexing device 430 may operate in the asynchronous mode based on the select signal. In the asynchronous mode, the signal on the signal line 434 (from the inverter circuit 420) passes through the multiplexing device 430 and subsequently passes through the inverter circuits 440, 450 and 460. The resulting signal is output to the signal trace 312 (i.e., the clock distribution network). The signal may also propagate back along the signal trace 322 to the inverter circuits 410 and 420 and be subsequently passed through the multiplexing device 430 since the multiplexing device 430 is still operating in the asynchronous mode. Accordingly, the select signal applied to the multiplexing device 430 may operate the clock generating circuit in either a synchronous mode or an asynchronous mode. Although not shown in FIG. 5, each of the clock generating circuits provided about the clock distribution network may include elements (such as inverter circuits and multiplexing devices) similar to the elements shown in FIG. 5. Each of the respective multiplexing devices may separately receive a similar select signal so as to provide the appropriate mode for the entire clock distribution network.

[0032] The multiplexing device 430 may be considered part of the ring oscillator. The multiplexing device 430 may be a pass-through element, which makes it logically passive for the loop. However, the multiplexing device 430 may contribute to delay (and therefore period) and Vcc sensitively of the delay of the loop.

[0033]FIG. 6 is a diagram showing the clock generating circuit of FIG. 5 along with elements to be clocked according to an example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. For ease of illustration, FIG. 6 only shows two clock generating circuits, namely a first clock generating circuit (in which the components are labeled A) and a second clock generating circuit (in which the components are labeled B). Both the first clock generating circuit and the second clock generating circuit may be similar to the clock generating circuit shown in FIG. 5 although other embodiments for a clock generating circuit are also within the scope of the present invention.

[0034] More specifically, the first clock generating circuit includes inverter circuits 410A, 420A, 440A, 450A and 460A as well as a multiplexing device 430A. Similarly, the second clock generating circuit includes inverters circuits 410B, 420B, 440B, 450B and 460B as well as a multiplexing device 430B. The first clock generating circuit may be coupled between the signal trace 312 and the signal trace 322. The second clock generating device may be coupled between a signal trace 314 (such as one of the signal traces 310) and the signal trace 322. FIG. 6 shows elements of the core 101 such as a D flip-flop circuit 510, logic 520 and a D flip-flop circuit 530. The D flip-flop circuits 510 and 530 may be clocked by the clock distribution network, such as clock signals on the signal trace 322. The logic 520 may include any type of latch, mechanism or state machine to perform a desired function in the core 101.

[0035]FIG. 7 is a diagram of a clock generating circuit according to an example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. In this embodiment, portions of the ring oscillator may be shared between a first clock generating circuit and a second clock generating circuit. More specifically, the inverter circuits 410 and 420 may be commonly used for both a first clock generating circuit (formed by at least the inverter circuits 440A, 450A and 460A) and a second clock generating circuit (formed by at least inverter circuits 440C, 450C and 460C). For ease of illustration, the multiplexing device for use in the ring oscillator is not shown in FIG. 7 although the multiplexing device may be provided after the inverter circuit 420 in one embodiment. The first clock generating circuit may output a clock signal on the signal trace 312 and the second clock generating circuit may output a clock signal on the signal trace 312.

[0036]FIG. 8 is a diagram of a clock generating circuit according to another example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. In this embodiment, elements of the ring oscillator may be shared between clock generating circuits. In this embodiment, adjacent ring oscillators may be “horizontally” coupled to share elements such as inverter circuits. For example, each of inverter circuits 502, 504, 506, 508 and 510 are shared between adjacent ring oscillators. This may help reduce wire delays. The clock signals may be output from the last stage of each ring oscillator circuit to a signal trace such as the signal trace 322. In this example, inverter circuits 512, 514, 516, 518 and 520 are the last stage of each ring oscillator. Although not shown in FIG. 8, multiplexing devices may be provided prior to the last inverter circuit of each ring oscillator circuit.

[0037]FIG. 9 is a diagram of a clock generating circuit according to another example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. In this embodiment, elements of the ring oscillators may be shared between clock generating circuits. In this embodiment, adjacent ring oscillators may be “horizontally” coupled to share elements. Ring oscillators may also be “vertically” coupled to share elements. The clock signals may be output from the last stage of the ring oscillator circuits to different signal traces. In this example, inverter circuits 542, 544, 546 and 548 are the last stage of each ring oscillator. Although not shown in FIG. 9, multiplexing devices may be provided prior to the last inverter circuit of each ring oscillator circuit.

[0038] As discussed above, the synchronous mode's signal may originate from a phase lock loop circuit located external from the core and be distributed with the clock distribution network. On the other hand, the asynchronous mode's signal may originate from itself. In a stopped state, all the ring oscillators may be de-asserted by an enable signal to one of the oscillator stages, which may be a NAND gate, for example. When an enable input is asserted high, then oscillation may begin. The enable signal to all the oscillators may be asserted simultaneously to start all the oscillators together.

[0039] More specifically, FIG. 10 shows a start/stop circuit according to an example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. The start/stop circuit operates to start driving the clock distribution network and to stop driving the clock distribution network. FIG. 10 shows a multiplexing device 610 to select between a synchronous mode and an asynchronous mode based on a synchronous/asynchronous select signal, which is also used as the select signal for the multiplexing device of each of the ring oscillator circuits. FIG. 10 also shows portions of the clock distribution network 620 (such as respective signal traces) coupled to inputs of NAND gates 640, 650 and 660. Other inputs to the NAND gates 640, 650 and 660 may be from signal lines between stages of each ring oscillator circuit. The output of each of the NAND gates 640, 650 and 660 is to one input of the multiplexing device (such as the multiplexing device 430) of each ring oscillator circuit. Accordingly, the circuit shown in FIG. 10 provides start/stop capabilities in the asynchronous mode.

[0040] In the synchronous mode, the PLL may drive the clock distribution network, and the core clock. When the asynchronous mode is selected, then the clock distribution network may be driven by the asynchronous start signal (shown as async_start), which is initially low and thus the core clock may be stopped. The async_start signal may be asserted to start the asynchronous mode oscillator. Since this signal travels down the clock distribution network, a simultaneous start may occur. This may be the same for subsequent stops/starts.

[0041]FIG. 11 is a graph showing the Vcc voltage signal on a die. As shown, the Vcc signal may vary due to reasons such as noise. The noise may impart Vcc modulation with peaks and valleys. FIG. 11 also shows a minimum Vcc(t) value that is located at a value no greater than the largest valley of the Vcc signal. In this arrangement, the frequency of the core would be at the lowest Vcc droop. Embodiments of the present invention allow the die to operate at an average Vcc(t) value. That is, the frequency of the core may track Vcc instantaneously. As clearly shown in FIG. 11, the average Vcc(t) value is higher than the minimum Vcc(t) value. This allows the performance to be based on the average power supply level rather than the minimum power supply level. This also reduces the need to minimize the power supply droop as in disadvantageous and costly arrangements.

[0042] Embodiments of the present invention have been described with regard to a method and apparatus to couple a core frequency to an instantaneous power supply such that the core frequency tracks the power supply (Vcc) to maintain functionality of the core logic in the face of severe supply noise. Embodiments of the present invention may include distributed oscillators to drive a common clock distribution network. The distributed oscillators may filter out the uncorrelated noise and respond to global supply noise. Embodiments of the present invention may further provide an asynchronous core I/O interface flexible enough to allow a wide range of instantaneous frequency ratios between the core and the I/O. An I/O ring around the core may run on a phase lock loop circuit synchronizing the I/O to the external world and thereby presenting a synchronous interface to the outside. An internal core phase lock loop circuit may drive the internal core clock in a synchronous mode to facilitate testing.

[0043] Embodiments of the present invention may provide a clock distribution network driven by a regular array of identical oscillators. This uniform structure may ensure that all the oscillators toggle simultaneously to produce a clock that reaches any point on the die at a coherent frequency with minimal skew between points. Each oscillator may include a ring oscillator made out of an odd number of inversion stages. These stages may be CMOS technology so that the elements of the ring oscillator track the power supply and temperature in the same fashion as the core datapath logic. The ring oscillator length may also be adjustable. The ring oscillator may be adjusted to a length that produces a period just long enough to ensure functionality of the worst-case core speedpath under any power supply and temperature condition. As Vcc-Vss increases, the worst speedpath may need less time to evaluate, and the oscillator frequency may proportionately increase to keep track. On the other hand, as Vcc-Vss decreases, the worst speedpath may require more evaluation time and that increase in time is provided by the slower oscillator frequency. As a result, the instantaneous oscillator frequency may track the worst speedpath, thus ensuring functionality over any (voltage, temperature) condition. Some amount of margin may counter any locally uncorrected noise in voltage and temperature.

[0044] One alternate mode may employ the PLL as a clock source. This allows testing of the microprocessor in the traditional synchronous mode, in which machine behavior is predictable on a cycle-by-cycle basis.

[0045] Embodiments of the present invention have been described with respect to a clock generating circuit and a clock distribution network. The clock distribution network is intended to include clock distribution grids and clock distribution trees and their equivalence. The clock generating circuit may be provided in areas (other than the core) that include any type of clock distribution network.

[0046] Any reference in this specification to “one embodiment”, “an embodiment”, “example embodiment”, etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. Furthermore, for ease of understanding, certain method procedures may have been delineated as separate procedures; however, these separately delineated procedures should not be construed as necessarily order dependent in their performance, i.e., some procedures may be able to be performed in an alternative ordering, simultaneously, etc.

[0047] Although embodiments of the present invention have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art. 

What is claimed is:
 1. A clock generating circuit comprising a plurality of distributed oscillators to drive a clock distribution network asynchronously with respect to an external clock.
 2. The circuit of claim 1, wherein each of the plurality of distributed oscillators comprises a plurality of inversion circuits.
 3. The circuit of claim 2, wherein each of the plurality of distributed oscillators comprises an odd number of inversion circuits coupled in series.
 4. The circuit of claim 1, wherein each of the plurality of distributed oscillators comprises a selecting circuit to select between a synchronous mode to drive the clock distribution network and an asynchronous mode to drive the clock distribution network.
 5. The circuit of claim 4, wherein the selecting circuit comprises a multiplexing device to select between the synchronous mode and the asynchronous mode.
 6. The circuit of claim 4, wherein in the synchronous mode, the clock distribution network is driven based on a signal output from a phase lock loop circuit and in the asynchronous mode, the clock distribution network is driven based on the plurality of distributed oscillators.
 7. The circuit of claim 1, wherein each of the plurality of distributed oscillators is powered by a power supply coupled to a power distribution network.
 8. The circuit of claim 7, wherein the distributed oscillators vary a frequency of driving the clock distribution network based on fluctuations of the power supply.
 9. The circuit of claim 1, further comprising a start/stop circuit to perform one of start driving the clock distribution network and stop driving the clock distribution network.
 10. The circuit of claim 1, wherein each of the plurality of distributed oscillators comprises a ring oscillator circuit.
 11. The circuit of claim 10, wherein an output of each ring oscillator circuit is coupled to the clock distribution network.
 12. The circuit of claim 1, wherein the clock generating circuit is provided within a core of an integrated circuit, and the external clock is external to the integrated circuit.
 13. A circuit comprising: a clock distribution network; and a device coupled to the clock distribution network to select between a synchronous mode and an asynchronous mode.
 14. The circuit of claim 13, wherein the device comprises a plurality of distributed oscillators to asynchronously drive the clock distribution network in the asynchronous mode.
 15. The circuit of claim 14, wherein the device comprises a plurality of selecting devices each coupled to the clock distribution network and each to select between the synchronous mode and the asynchronous mode.
 16. The circuit of claim 15, wherein each of the plurality of selecting devices comprises a multiplexing device to select between the synchronous mode and the asynchronous mode.
 17. The circuit of claim 16, wherein in the synchronous mode, the clock distribution tree is driven based on a signal output from a phase lock loop circuit and in the asynchronous mode, the clock distribution network is driven based on the plurality of distributed oscillators.
 18. The circuit of claim 14, wherein each of the plurality of distributed oscillators comprises a plurality of inversion circuits.
 19. The circuit of claim 18, wherein each of the plurality of distributed oscillators comprises an odd number of inversion circuits coupled in series.
 20. The circuit of claim 14, wherein each of the plurality of distributed oscillators is powered by a power supply coupled to a power supply distribution network.
 21. The circuit of claim 20, wherein the distributed oscillators vary a frequency of driving the clock distribution network based on fluctuations of the power supply.
 22. The circuit of claim 13, further comprising a start/stop circuit to perform one of start driving the clock distribution network and stop driving the clock distribution network.
 23. The circuit of claim 14, wherein each of the plurality of distributed oscillators comprises a ring oscillator circuit.
 24. The circuit of claim 23, wherein an output of each ring oscillator circuit is coupled to the clock distribution network.
 25. The circuit of claim 13, wherein the circuit is provided within a core of an integrated circuit.
 26. The circuit of claim 13, further comprising a latch element to receive a clock signal from the clock distribution network.
 27. A circuit comprising: a clock distribution network; a device coupled to the clock distribution network to select between a synchronous mode and an asynchronous mode; and an element to receive a clock signal from the clock distribution network.
 28. The circuit of claim 27, wherein the device comprises a plurality of distributed oscillators to drive the clock distribution network in the asynchronous mode.
 29. The circuit of claim 28, wherein each of the plurality of distributed oscillators comprises a plurality of inversion circuits.
 30. The circuit of claim 28, wherein each of the plurality of distributed oscillators is powered by a power supply coupled to a power supply distribution network.
 31. The circuit of claim 30, wherein the distributed oscillators vary a frequency of driving the clock distribution network based on fluctuations of the power supply.
 32. The circuit of claim 27, further comprising a start/stop circuit to perform one of start driving the clock distribution network and stop driving the clock distribution network.
 33. The circuit of claim 28, wherein each of the plurality of distributed oscillators comprises a ring oscillator circuit.
 34. The circuit of claim 33, wherein an output of each ring oscillator circuit is coupled to the clock distribution network.
 35. The circuit of claim 27, wherein the circuit is provided within a core of an integrated circuit.
 36. A clock generating circuit comprising: a plurality of ring oscillator circuits each to couple to a clock distribution network and each to supply a respective output signal to the clock distribution network, each ring oscillator circuit including a selecting device to select between an asynchronous mode and a synchronous mode.
 37. The circuit of claim 36, wherein each of the plurality of ring oscillator circuits comprises a plurality of inversion circuits.
 38. The circuit of claim 36, wherein each of the plurality of ring oscillator circuits is powered by a power supply coupled to a power supply distribution network.
 39. The circuit of claim 38, wherein the ring oscillator circuits vary a frequency of driving the clock distribution network based on fluctuations of the power supply.
 40. The circuit of claim 36, further comprising a start/stop circuit to perform one of start driving the clock distribution network and stop driving the clock distribution network.
 41. The circuit of claim 38, wherein an output of the each ring oscillator circuit is coupled to the clock distribution network.
 42. The circuit of claim 36, wherein the clock generating circuit is provided within a core of an integrated circuit.
 43. A method of generating a clock signal comprising: driving a clock distribution network in a synchronous mode; and driving the clock distribution network in an asynchronous mode.
 44. The method of claim 43, wherein driving the clock distribution network in the asynchronous mode comprising utilizing a network of distributed oscillators about the clock distribution network.
 45. The method of claim 44, wherein each distributed oscillator comprises a plurality of inversion circuits.
 46. The method of claim 44, wherein each distributed oscillator is powered by a power supply coupled to a power supply distribution network.
 47. The method of claim 46, further comprising varying a frequency of the clock distribution network based on fluctuations of the power supply.
 48. The method of claim 43, further comprising start driving the clock distribution network in the asynchronous mode and stop driving the clock distribution network in the asynchronous mode.
 49. The method of claim 44, wherein each distributed oscillator comprises a ring oscillator circuit.
 50. The method of claim 43, wherein driving the clock distribution network in the synchronous mode comprises utilizing an output of a phase lock loop circuit that provides a synchronous clock signal based on a clock source external to the clock distribution network.
 51. The method of claim 43, further comprising selecting the asynchronous mode prior to driving the clock distribution network in the asynchronous mode.
 52. The method of claim 43, wherein driving the clock distribution network in the synchronous mode and driving the clock distribution network in the asynchronous mode comprises utilizing a selecting device located with a core of an integrated circuit. 